Low toxicity solvent composition

ABSTRACT

A low-toxicity solvent composition having broad-spectrum applicability. A polar solvent component is provided by an alicyclic carbonate and a non-polar solvent component is provided by a terpene, with a short-chained non-ionic surfactant being provided for coupling the alicyclic carbonate and terpene in a homogeneous, single-phase solution. The composition may be 25-60% propylene carbonate, 1% (or less)-20% d-limonene, and 40-60% tripropylene glycol methyl ether. This solvent composition is non-toxic to personnel, and presents minimal environment hazards. Among its many applications are paint equipment clean-up, cleaning and reclamation of silkscreens, cleaning of offset printing rollers, cleaning of aluma-printing surfaces and equipment, as well as general purpose clean-up of greases, oils, paints, inks, and so forth.

This is a continuation of copending application Ser. No. 07/839,854filed on Feb. 21, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to liquid organic solvents, andmore particularly, to such a solvent which effectively dissolves andremoves a wide range of materials, yet which is also comparativelynon-hazardous to the environment and personnel.

2. Background

Liquid solvent compositions are widely used throughout modern industry.Just a few of the many applications for such solvent compositionsinclude the following processes: oil and grease removal; cleaning ofpaint guns and lines; stripping paint; washing paint rollers; cleaningand reclamation of silk screens; aluma-printing; deglazing, ink removal,and roller washing in printing processes; lacquer washing.

As is well known to those skilled in the art, the many various materialswhich must be solubilized in these applications differ greatly in howeffectively they can be dissolved by different solvent materials. Forexample, the materials may be characterized as "polar" or "non-polar"(or somewhere in between) as a result of their molecular structure; ithas been found that as a general rule, "like dissolves like", so thatmaterials having a generally polar character will tend to be mosteffectively solubilized by solvents also having a polar character (i.e.,those solvents which rely largely on their electrical dipolecharacteristics for their solvent action), while materials of anon-polar character are usually more effectively solubilized by solventswhich also have non-polar characteristics (i.e., those solvents whichwork primarily on the basis of their dispersion forces).

While it would thus in some respects be "ideal" to mate the materialwhich is sought to be removed with a solvent which is matched to it interms of its polar/non-polar characteristics, this is frequently notpossible, or is at least impractical. For one thing, coatings and othermaterials may themselves be made up of both polar and non-polarconstituents, and it is necessary for the solvent to be able to act onboth of these in order to successfully remove the material. Furthermore,it is simply desirable from an economic and convenience standpoint tohave a "general purpose" solvent available which can be relied on toperform many different cleaning tasks involving a wide variety ofmaterials.

Unfortunately, relatively few solvents formulations have been foundwhich are capable cleaning up a broad spectrum of materials having thevaried characters discussed above. In general, the search for suchgeneral purpose solvents has focused on compounds which exhibit bothpolar and non-polar characteristics in a single molecule; for example,some molecules are essentially "polar" at one end, and "non-polar" atthe other, in terms of their solvent characteristics. Some of thesematerials (e.g., fatty acids and the like, which are used in detergentmixtures) are characterized by a long-chained molecular structure, andare generally unsuitable for use in many industrial applications, due tothe excessive residue which they leave behind, and the amount of rinsingor other secondary washing which is necessary to remove this.

On the other hand, a handful of organic compounds have been identifiedwhich have been successfully used as broad spectrum solvents in hightechnology industries, such as the aerospace and electronic industries,as well as in more commonplace applications. Unfortunately, the greatmajority of these have ultimately been found to present undesirabletoxicologies and serious hazards to the environment; examples of thesecompounds include methylene chloride and methyl ethyl ketone (MEK), aswell as toluene, xylene, and other aromatics, may of which include theadditional hazard of high flammability. For example, although MEK haslong been considered a satisfactory solvent from the standpoint ofcleaning effectiveness, there is a growing concern that the toxicity andflammability of MEK exposes users to unnecessary risks. Also, becauseused MEK is considered a threat to the environment, and so is classifiedas a hazardous waste, the expense associated with the safe disposal ofMEK is on the order of 5-10 times greater than the amount which the userinitially pays for the solvent. Moreover, because of its relatively highvapor pressure, the lose of MEK to the atmosphere during use isexcessive, necessitating the use of large and expensive collectionsystems such as vacuum hoods.

Because of the concern for the safety, health, and environmental hazardswhich these known organic solvents thus present, both the federal andstate governments are promulgating increasingly stringent criteria whichsolvent users must comply with. For instance, the California StateLegislature limits the use of volatile solvents by requiring that theyhave a vapor pressure below about 45 mmHg at 20° C. In addition,regulations require that solvents be disposed of in a manner that willnot adversely effect the environment; for may users of such solvents,this disposal generally translates into increased operating costs, asnoted above.

For the above reasons, a primary consideration for many users of organicsolvents has become the toxicity of a particular solvent mixture, andalso the hazards which it presents to the environment. This has lead toa number of attempts to find safe substitutes for the hazardous organicsolvents which have been used in the past. As an example, methylenechloride has been widely used in industry, especially for formulatingpaint strippers, lacquer removers, and paint clean-up systems, but itsuffers from high volatility which leads to excessive evaporation,contributing to worker exposure and environmental pollution. Attemptshave consequently been made to replace methylene chloride using various,safer organic solvents, but for the most part these efforts have notyielded solvent compositions which are sufficiently effective or quickin action to gain acceptance, and, furthermore, many of the proposedsubstitutes have proven to costly to be economically feasible. Forexample, n-methyl-pyrrolidone (NMP) has sometimes been found to be asuitable substitute for MEK or methylene chloride in terms of itssolvent abilities, and it exhibits a very low volatility whichdrastically reduces the flammability hazard and evaporative losses.However, both the cost of NMP renders it use prohibitive in theconcentrations which are necessary to make many of the proposedformulations perform effectively as solvents. Furthermore, NMP isexcessively harsh for many applications, in that it will cause damage tothe underlying substrate; for example NMP can cause severe deteriorationof rubber and plastics, such as PVC. It also tends to cause irritationand defatting of user's hands.

Accordingly, a need exists for a substantially non-toxic solventcomposition which exhibits superior cleaning ability when applied to avariety of substances, and which exhibits low flammability andrelatively low vapor pressure so as to limit evaporative losses.Furthermore, there is a need for such a solvent which safely degrades inthe environment and in biological systems, and which is alsosufficiently inexpensive to be economical for large scale use.

SUMMARY OF THE INVENTION

The present invention has solved the problems cited above and is a lowtoxicity solvent composition, this comprising broadly (a) an alicycliccarbonate for providing the composition with a polar solvent component;(b) a terpene for providing the composition with a non-polar solventcomponent; and (c) a short-chained, non-ionic surfactant for couplingthe alicyclic carbonate and the terpene in a homogeneous, single-phasesolution.

The alicyclic carbonate may be selected from the group consisting ofpropylene carbonate and ethylene carbonate, and is most preferablypropylene carbonate. The propylene carbonate may be present in thecomposition in an amount from about 25% to about 60% by weight.

The terpene which is used in the composition may be a monoterpene, andthis may be selected from the group consisting of d-limonene andl-limonene, with d-limonene being preferred. The d-limonene may bepresent in an amount from about 1% or less to about 20% by weight.

The short-chained non-ionic surfactant may be a glycol ether. This maybe a glycol ether which is selected from the group consisting ofpropylene-based and ethylene-based glycol ethers, with thepropylene-based glycol ethers being preferred. Tripropylene glycolmethyl ether is most preferred, and this may be present in thecomposition in an amount from about 40% to about 60% by weight.

DETAILED DESCRIPTION OF THE INVENTION

As an overview, it has been found that a particularly effective liquidsolvent is provided when a liquid mixture of an alicyclic carbonate anda terpene is formed, this being stabilized in a single phase solution bya small-chained, non-ionic surfactant, such as a glycol ether. Forexample, it has been found that a preferred mixture of propylenecarbonate, d-limonene, and tripropylene glycol methyl ether provides aliquid solvent which exhibits superior cleaning properties with respectto a wide variety of materials, a virtual absence of toxicity, and a lowvapor pressure which greatly reduces evaporative loss and personnelexposure in use. Furthermore, this mixture is economical to produce,which advantage is enhanced by the fact that the stability of themixture and the low evaporative loss greatly extends the service life ofa given amount of this solvent.

As was noted above, a known (but extremely hazardous) organic solventwhich has the ability to solubilize a wide range of materials is methylethyl ketone, and this ability is believed to stem from the fact thatMEK combines both polar and non-polar characteristics in a singlemolecule. The present invention, in turn, is intended to produce asolvent material which, on a macro scale, may mimic some of thepermanent electrochemical characteristics which are exhibited by MEK ina single molecule. Accordingly, the cyclic carbonate provides the polarcomponent of the solution, and the terpene provides the non-polarcomponent. The glycol ether, in turn, serves as a "coupling agent" whichmaintains the two solvent portions (i.e., the polar and non-polarportions) in a homogeneous, single phase solution; this enables thesolvent system to develop a synergism which permits it to solubilizematerials having solvent-related characteristics which lie either at orbetween these extremes in terms of polarity. The net result is that thiscomposition has been found to exhibit a solvent ability whichout-performs any of the three components used separately; this increasein performance has been observed with respect to numerous differentmaterials, and especially with respect to solvent-based paints.

As was noted above, the polar portion of the solvent composition isprovided by the alicyclic carbonate compound, these compounds being ofthe formula: ##STR1## wherein R₁ and R₂ are selected from the groupconsisting of hydrogen, methyl, or ethyl. Examples of alicycliccarbonate compounds having this formula which are preferred for use inthe composition of the present invention include propylene carbonate andethylene carbonate. As is known to those skilled in the art, the polarcharacter of those carbonate compounds stems from the strong positivepolarity which the oxygen bonding imparts to the carbonate end of themolecule.

The reasons the alicyclic carbonate compounds have been foundadvantageous for use in the composition of the present invention appearto be at least twofold. Firstly, the cyclic structure of these compoundsrenders the molecules significantly more compact than theirstraight-chained equivalents, and this greatly reduces steric(mechanical) hindrances between the molecules of the solvent compositionand those of the material which is sought to be solubilized. Forexample, the compact nature of the cyclic versions of the carbonatesappears to make it much easier for those to penetrate the micro-porestructure which exists at the solvent interface with coatings of paint.Secondly, it has been found that these cyclic carbonates are much morereadily maintained in a homogeneous, single phase solution with thecyclic terpenes which are the preferred non-polar component of thecomposition, when these are combined with the glycol ether couplingagent, this apparently being due to "stacking" of the cyclic structuresof the carbonate and the terpene in the solution.

Of the two preferred cyclic carbonates noted above, the propylenecarbonate is most preferred in the solvent composition of the presentinvention, in that, while both ethylene and propylene carbonate degradequickly and safely in the environment, and both exhibit desirably lowvapor pressures (as will be discussed further below), the propylenecarbonate is considerably safer from the standpoint of personnelexposure, inasmuch as it degrades to safe intermediates and end productswithin the human metabolic system, while this is not true of ethylenecarbonate. In fact, the safety of propylene carbonate is attested to bythe fact that it has been widely used is cosmetics. Propylene carbonatesuitable for use in the composition of the present invention isavailable from several sources; for example, Texaco Chemical Company,Thousand Oaks, Calif., supplied suitable propylene carbonate under thebrand name TEXACAR^(*) PC™.

Turning now to the non-polar constituent of the composition, this (aswas noted above) is provided by a suitable terpene. As is known to thoseskilled in the art, such non-polar solvents generally rely more on theirdispersion forces for their solvent action, rather than on electricaldipole characteristics, as is the case with the polar compounds.

Terpenes are hydrocarbons often found in essential oils, resins, andother vegetable aromatic products, and, in general, are perceived to bepolymers of a 5-carbon moiety referred to as an isoprene unit. Relatedto the terpenes are the hemiterpenes (C₅ H₈), sesquiterpenes (C₁₅ H₂₄),diterpenes (C₂₀ H₃₂), and the polyterpenes (n(C₁₀ H₁₆)). Themonoterpenes (C₁₀ H₁₆) which are preferred for use in the solventcomposition of the present invention are primarily of plant origin, avery large number of these having been isolated and characterized; manyhave long been used in perfumes and medicines, and consequently many ofthese present significant advantages in terms of safety for personnelexposure. Examples of suitable terpenes include the di-pentenesd-limonene and l-limonene, and also pinene, terpinene, and terpinolene.

As was noted above, the cyclic terpenes are generally preferred for usein the solvent composition of the present invention, due to theircompact molecular structure and ability to "stack" with the cycliccarbonate component of the solution, the most preferable of these beingd-limonene, a by-product of the citrus industry; it is also believedthat the double-bonded structure of d-limonene provides a flatter ringconfiguration which more effectively emulates the solventcharacteristics of the aromatic hydrocarbons which are sought to bereplaced. This compound is derived in various amounts from the rinds orpeels of oranges, grapefruits, and other citrus fruits; it safelybiodegrades in the environment, and does not present a personnel hazardin terms of exposure, this material having in fact been used in variousfood products. The structure for d-limonene is given below: ##STR2## Anextensive discussion of d-limonene, and its derivation from varioussources, is presented in a book by J. W. Kesterson, R. Hendrickson, andR. J. Braddock, entitled "Florida Citrus Oil", and published in December1971 by Agricultural Experiment Station, Institute of Foods andAgricultural Sciences, University of Florida, Gainesville, Fla.

The d-limonene employed in the compositions of the present invention canbe of a relatively impure grade without causing significant degradationof the solvent capabilities of the resultant solution. However, someresearchers believe that the presence of significant amounts ofimpurities in d-limonene speeds the formation of decomposition productsto which the skin of some people's hands may be sensitive, and so morerefined grades of d-limonene are preferable from this standpoint. Also,the more highly refined grades of d-limonene lack the citrus odor whichis characteristic of the material, but the odor itself is generally notconsidered offensive.

The final component of the solvent composition is the "coupler" whichpermits the two, dissimilar solvent portions to exist together in ahomogeneous, single phase solution. Without the inclusion of such a"coupler" (or "coupling agent"), it is simply not possible to get thepolar carbonate and non-polar terpene to stay in the solution together;these compounds very strongly tend to "bead" when any attempt is made tomix them together in the absence of a coupler, and they will quicklyseparate out from one another, even after vigorous mixing and agitation.

A characteristic of effective coupling agents has been found to be thatthey exhibit both polar and non-polar characteristics in the samemolecule; in other words, each molecule is part polar and partnon-polar, so as to essentially provide a link between the polar andnon-polar molecules in the solution. In this regard, it has been foundthat small-chained, non-ionic surfactants are suitable for use ascoupling agents in the solvent composition of the present invention, andthat glycol ethers and their acetates are preferable for this purpose.Suitable glycol ethers and their acetates have the structure:

    (R.sub.4 --O).sub.n2 --(R.sub.3 --O).sub.n1 --R.sub.5

wherein _(n1) and _(n2) are the numerals 1-3, and wherein R₃ is ahydrocarbon radial having 2-3 carbon atoms, R₄ is a hydrogen or ahydrocarbon radical having 1-4 carbon atoms, and R₅ is a hydrogen or ahydrocarbon radial having 1-4 carbon atoms.

Both ethylene- and propylene-based glycol ethers have been foundparticularly effective as coupling agents for the solvent composition ofthe present invention, and suitable examples of these two families arelisted below:

Propylene-Based Glycol Ethers

Propylene Glycol

Methyl Ether

Dipropylene Glycol

Methyl Ether

Tripropylene Glycol

Methyl Ether

Propylene Glycol

Methyl Ether Acetate

Dipropylene Glycol

Methyl Ether Acetate

Propylene Glycol

n-Butyl Ether

Propylene Glycol

n-Butyl Ether

Propylene Glycol

t-Butyl Ether

Ethylene-Based Glycol Ethers

Ethylene Glycol

n-Butyl Ether

Diethylene Glycol

n-Butyl Ether

Triethylene Glycol

n-Butyl Ether

Diethylene Glycol

Methyl Ether

For reasons essentially similar to those which were discussed above withrespect to the advantages of propylene carbonate relative to ethylenecarbonate, the propylene-based glycol ethers are generally preferredover the glycol-based glycol ethers, since the propylene-based compoundsdegrade safely in a living system. In particular (as is also the casewith propylene carbonate), the main metabolite of the propylene-basedglycol ethers is propylene glycol, which is used extensively incosmetics and as a food additive. Most preferred for use in thecomposition of the present invention is tripropylene glycol methylether, because of its very low vapor pressure. The vapor pressure oftripropylene glycol methyl ether is lower than that of either thepropylene carbonates of the d-limonene, with the result that the glycolether is the last to evaporate off; this ensures that the coupling agentwill always be present in the composition so as to prevent the carbonateand terpene from separating out, this relationship being maintainedregardless of any evaporative losses which may occur over a long periodof use. The ethylene-based glycol ethers, by contrast, while they alsobiodegrade well in the environment, are less desirable from thestandpoint of personnel exposure, inasmuch as some are readily absorbedthrough the skin and do not degrade as safely within living systems.

EXAMPLE FORMULATIONS

The novel solvent compositions of the present invention will be morefully understood from a consideration of the following examples, whichillustrate preferred embodiments. It is to be understood, however, thatthese examples are given by way of illustration and not of limitation.

EXAMPLE I

A solvent composition was prepared by simple mixing of the following,preferably always maintaining a sufficient amount of the glycol ether inthe solution to prevent beading of the other two components:

    ______________________________________                                               Component    % (By Weight)                                             ______________________________________                                        (a)      Propylene Carbonate                                                                          25                                                             (CAS #108-32-7,                                                               1,3-Dioxolan-2-one                                                            methyl)                                                              (b)      d-Limonene     17                                                             (CAS #5989-27-5,                                                              4-isopropenyl-1-                                                              methylcyclo hexene)                                                  (c)      Tripropylene Glycol                                                                          58                                                             Methyl Ether                                                                  (CAS #25498-49-1                                                     ______________________________________                                    

This particular formulation (which has been designated "EP921") haswidespread applicability, and has been found particularly effective foruse in cleaning up guns and lines have been used for applying paint andother coatings, and also for the deglazing of rollers which are used inoffset printing.

EXAMPLE II

A solvent composition was prepared containing:

    ______________________________________                                               Component    % (By Weight)                                             ______________________________________                                        (a)      Propylene Carbonate                                                                          60                                                             (CAS #108-32-7,                                                               1,3-Dioxolan-2-one,                                                           methyl)                                                              (b)      d-Limonene     1 (or less)                                                    (CAS #5989-27-5,                                                              4-isopropenyl-1-                                                              methylcyclo hexene)                                                  (c)      Tripropylene Glycol                                                                          39                                                             Methyl Ether                                                                  (CAS #25498-49-1)                                                    ______________________________________                                    

This formulation is somewhat more specific in its applicability thanthat of Example I, this having been found useful primarily for thecleaning and reclamation of silkscreens, and in the clean-up ofaluma-printing surfaces and equipment. This is the result of reducingthe amount of d-limonene to 1% or less, which tends to enhance the polaraspect of the solvent mixture. Nevertheless, it has been found that,even at these relatively low levels (e.g., 0.5-1%), the d-limoneneenables the synergism to be developed with the propylene carbonate suchthat the effective solvent action of this mixture is significantlygreater than that which would be exhibited by the mixture if itcontained only the propylene carbonate and the glycol ether.

Other exemplary formulations in accordance with the present inventionmay be prepared as follows:

EXAMPLE III

    ______________________________________                                                           by weight                                                  ______________________________________                                        (a)       Propylene Carbonate                                                                          36.2%                                                (b)       d-limonene     7.4%                                                 (c)       Tripropylene Glycol                                                                          56.4%                                                          Methyl Ether                                                        ______________________________________                                    

EXAMPLE IV

    ______________________________________                                                           by weight                                                  ______________________________________                                        (a)       Propylene Carbonate                                                                          40.3%                                                (b)       d-limonene     4%                                                   (c)       Propylene Glycol                                                                             55.7%--                                                        Ether Acetate                                                       ______________________________________                                    

The compositions provided by the exemplary formulations given above havebeen found to be highly effective solvents which are non-corrosive tometal substrates. The design of EP921 creates material which mimics thesolubility parameters and, more importantly, creates near congruencewith the solubility vector, of methyl ethyl ketone. The radii ofinteraction have been artificially expanded, and an enhances spectrum ofwettability created, so as to enlarge the scope of paints and resinswhich can be cleaned with this material. However, in contrast with MEK,the material is safe for human exposure, and poses minimal wastedisposal problems. The blends were consequently found to be safe andeffective substitutes to replace MEK and MEK/toluene blends, especiallyfor paint gun and line clean-up, and for silkscreening applications. Itwas also found that the EP921 formulation provides an effectivereplacement for the methylene chloride, toluene, and 1,1,1trichloro-ethane blends which have traditionally been used in offsetprinting processes to deglaze the ink and metering rollers. On a morecommonplace level, this solvent composition has been found to producegood results when used for the removal of glues and adhesives, as a mildpaint remover, as a grease and oil remover, and for removal of feltmarker markings and machinist's blue dye, thus demonstrating itseffectiveness as a general purpose cleaner. Final rinsing to removeresidual solvent is easily achieved with warm water, followed by an airblast to dry the part if necessary.

Analytical Results and Field Testing

A thorough product analysis and extensive field testing was conductedwith respect to the exemplary solvent compositions having theformulations set forth in Examples I and II above. For example, EP921(i.e., the formulation of Example I above) has been found to exhibit thefollowing relevant characteristics:

    ______________________________________                                        TECHNICAL INFORMATION                                                         ______________________________________                                        Boiling Point:      >340° F.                                           Vapor Pressure (MMHG)                                                                             .2 at 25° C.                                       Vapor Density Air = 1                                                                             4.7                                                       Specific Gravity H.sub.2 O = 1                                                                    .98                                                       % Volatile by weight                                                                              17%                                                       Volatile component  167 grams per liter                                       Overall Solubility Parameter                                                                      9.3                                                       London Dispersion Forces                                                                          8.1                                                       Polarity            3.2                                                       Hydrogen Bonding    3.2                                                       ______________________________________                                    

A review of the components contained in these formulations reveals thatthese lie outside of the RCRA (Resource Conservation And Recovery Act of1976) Hazardous Waste Regulations, and so the very expensive wastedisposal costs which are associated with MEK and similarly hazardouscompounds are avoided; similarly, none of the components are listedunder CERCLA (Comprehensive Environmental Response, Compensation, andLiability Act of 1980--"Superfund"). Furthermore, none of thesecomponents (assuming use of the preferred propylene-based components)are listed in SARA Title III, §302 or 313, and so are not consideredtoxic chemicals in the workplace. Still further, the low VolatileOrganic Compounds (VOC) content and high flash point of thesecompositions contribute to making them ideal solvents for functioningwithin the modern regulatory climate. The very low vapor pressure (seetable above) minimizes evaporative losses to the atmosphere, and, infact, the emission limitations which are thus inherent in thisformulation may qualify it as a Best Available Control Technology (BACT)for regulatory purposes, by comparison with known formulations usingvapor collection systems. For example, even without the use of a vaporcollection system, the losses to the atmosphere from the EP921formulation compare very favorably with the use of MEK in conjunctionwith a vapor collection system having 99.7% efficiency (which efficiencywould be difficult to attain in practice).

These advantages were confirmed by theoretical analyses of the solventcompositions, as well as by extensive field testing. As regards thetheoretical analysis, this was conducted using two models, the firstassuming a closed system and the second assuming an open system, withreal world performance expected to fall somewhere between the resultsderived from these two models. The first model assumed that equilibriumhad been reached between a liquid and vapor phase in an enclosedenvironment. Under this condition, the amount of material lost to thegas phase corresponds to the vapor pressure of each of the two chemicalsbeing compared. The vapor pressure of MEK was determined to beapproximately 70 mmHg, while (as was noted in the table above) the vaporpressure of the EP921 formulation of the present invention was found tobe 0.2 mmHg. Based upon these determinations, and assuming that bothmaterials exhibit similar vapor and liquid behaviors, it was calculatedthat the amount of MEK lost to evaporation would be roughly 350 timesgreater than that of the EP921 solvent.

To then compare the two liquids in a non-equilibrium open system, it wasnecessary to use evaporation rates in the calculations. The evaporationrate given for MEK ranges from 3.8 to 5.7 (butyl acetate=1.0), whereasthe evaporation rate for the EP921 version of the solvent of the presentinvention was calculated to be less than 0.02. In order to be on theconservative side, the least favorable numbers relative to the EP921solvent formulation were used in making this calculation; accordingly,MEK is assumed to have an evaporation rate of 3.8, and EP921 was assumedto have an evaporation rate of 0.02. From these evaporation rates, itwas determined that it would take the EP921 solvent 190 times longerthan MEK to evaporate the same quantity of liquids. In other words, ifit took 30 minutes to lose 90% of the MEK from an open container, itwould theoretically take 4 days to lose the same amount of EP921.

Of course, actual consumption in real world operations is influenced byvarious other factors besides vapor pressures and theoreticalevaporation rates. Actual consumption creates are significantly effectedby drag-out, agitation longevity of cleaning power, reclamation efforts,and individual site practices. Accordingly, two site tests were carriedout to determine the performance of the solvent compositions of thepresent invention relative to that of MEK.

The first field test was conducted using the "EP921" solvent given byExample I above. This testing was conducted in a large pulp and papermill having 17 full-time painters engaged in on-site maintenancepainting. Prior to adopting the EP921 solvent as a substitute for MEK incleaning painting equipment, this facility routinely consumed twelve55-gallon drums of MEK per month for this purpose. Since adopting thelow-vapor pressure EP921 solvent as a replacement for the MEK,consumption (of the EP921) has fallen to between two and four drums permonth. Inasmuch as this facility did not practice any reclamation orlife-extending procedures with respect to the solvent during this test,this usage difference is most probably entirely attributable to thedifferences in cleaning power and evaporation rate between EP921 and theMEK which it has replaced.

Similarly, site testing was carried out using the solvent compositionwhich is set forth in Example II above. This formulation, which there isrelatively low amount of d-limonene, is especially well-suited for usein cleaning and reclamation of silkscreens, and so has been assigned thedesignation of "Silkscreen B".

The site selected was a facility using an aluma printing process toscreen permanent printing on aluminum sheeting. This shop's priorconsumption was 7000 pounds of MEK per month, just to support thisprocess. After switching to the low-vapor pressure solvent in accordancewith the present invention, the site's consumption of solvent diminishedto 1600 pounds per month. Simultaneously, a several-fold increase inproductivity was achieved: with the MEK process, productivity was 1250parts per week; after switching the process to the "Silkscreen B"solvent composition of the present invention, productivity increased to4000 parts per week. The lower productivity experienced when using MEKappears to have been related to the frequent need to shut the systemdown and replace the dirty MEK; "Silkscreen B" lasts far longer in thisprocess, resulting in far fewer change-outs and much less down time.

As an additional benefit of the low vapor pressure and evaporation rateswhich are exhibited by the solvent of the present invention relative toMEK, it has been found that this greatly reduces the likelihood that thesolubilized material will re-adhere to or re-deposit on the cleanedsurface to evaporative losses of the solvent.

The present invention thus provides a liquid solvent mixture which islow in toxicity, and presents an effective and environmentallybeneficial alternative to conventional cleaning solvents like MEK andmethylene chloride. The preferred mixture of propylene carbonate,d-limonene, and tripropylene glycol methyl ether out-performs mostconventional solvents, without harming the substrate, be it ceramic,glass, metal, or plastic. The field experience with the solvent mixtureof the present invention demonstrated that this is not only a safe andeconomical solvent, especially from the standpoint of regulatoryrequirements, but that it is a surprisingly effective solvent for manyindustrial cleaning applications, and that its longevity in useincreases its economic advantages.

The solvent of the present invention can include other additives toaddress specific cleaning problems, and provide further improved solventeffect for particular applications. The selection of the particularadditives and the amounts used should be consistent with the objectiveof providing a solvent with a low-vapor pressure which is essentiallynon-toxic and safe in terms of environmental hazards. For example,thickening agents may be added to the solution if so desired.

One of ordinary skill, after reading the foregoing specification and theappended claims, will be able to effect various changes, substitutionsof equivalents, and other alternatives without departing from the broad,inventive concepts discussed herein. Accordingly, the claims should beconstrued broadly in light of the description so as to include alldescried embodiments and their equivalents, and should only be limitedas required by the relevant prior art.

What is claimed is:
 1. A low toxicity solvent composition consistingessentially of:(a) propylene carbonate in an amount from about 25% toabout 60% by weight of said composition; (b) d-limonene in an amountfrom about 1% to about 20% by weight of said composition; and (c)tripropylene glycol methyl ether in an amount from about 40% to about60% by weight of said composition.
 2. A low toxicity general purposesolvent composition consisting essentially of:(a) propylene carbonate inan amount of about 25% by weight of said composition; (b) d-limonene inan amount of about 17% by weight of said composition; and (c)tripropylene glycol methyl ether in an amount of about 58% by weight ofsaid composition.
 3. A low toxicity solvent composition consistingessentially of:(a) propylene carbonate in an amount of about 60% byweight of said composition; (b) d-limonene in an amount of about 1% ofsaid composition; and (c) tripropylene glycol methyl ether in an amountabout 39% of said composition.